578 LILLIE. 



EXPLANATION OF PLATE. 



Fig. I. Four-cell stage from the apical pole, i-i = First plane of segmenta- 

 tion. 2-2 = Second plane of segmentation. ;«././. = Median longitudinal plane 

 of future larva, /"^jj/. = posterior ; /^«^. = Anterior. 



Fig. 2. Eight-cell stage from the apical pole. «,' b,^ c,' «" the four apical 

 micromeres derived from A, B, C, and D, respectively. 



Fig. 3. Seventeen-cell stage from the vegetative pole. A, B, C, and D the 

 four macromeres. a,^ b^ c,^ d^ the second generation of micromeres. d^ or X is 

 the first somatoblast. x ' is derived from X as indicated by the position of the 

 nuclei. The spindle in D later separates d^ to the left. 



Fig. 4. Vegetative pole of forty-six-cell stage. ^, x,'-' x,'-^ x,^ jr* = products 

 of first somatoblast X. yI/= second somatoblast or mesoderm proteloblast, 

 derived from D. K= larval mesoblast, derived from a.^ y^ and y'^ are products 

 of Y. A, B, C, D = Entomeres. 



Fig. 5. Sagittal section of young gastrula. ^/. := blastopore ; ^«A = ento- 

 derm ; s. g.= cells of shell-gland ; X = products of the first somatoblast X ; 

 M^ mesoderm teloblast of one side which has budded off one cell anteriorly. 

 y= larval mesoblast of one side. 



Fig. 6. Sagittal section of an older stage. Ent. = entoderm ; s. g. = shell- 

 gland ; X, M, and Y as above. Oes. = first indications of oesophageal 

 invagination. 



